This invention relates to methods for cementing wells penetrating permeable, fluid-containing strata. More particularly, this invention relates to methods for cementing a wellbore that penetrates permeable strata which contain corrosive fluids so as to prevent casing corrosion and erosion by contact with formation fluids and well additives.
In the drilling of wells, for example oil wells, wells penetrating sources of geothermal energy, and the like, it is common practice to use a cement to hold a well casing in place and to selectively block or plug strata penetrated by the well to prevent loss of drilling muds and fluids. The cement, when so used, is pumped as a slurry into the annular space between the bore of the well and the casing and then is permitted to cure into an impermeable hardened mass.
To resist the temperatures and pressures encountered in wells, cements have been developed to provide maximum compressive strength and resistance to thermal degradation. Usually the cements are conventional Portland-type cements to which have been added any of a number of additives such as mica, blast furnace slag, alumina or special reactive sands designed to improve the mechanical strength and the thermal or chemical resistance of the set and hardened cement. However satisfactory these cements have proven for conventional well operations, in geothermal wells where temperatures in excess of 400.degree. F. may be encountered, they have quickly deteriorated, suffering increased porosity, permeability, and loss of compressive strength sufficient to lead to blowouts.
Accordingly, oil well cements used in geothermal wells, or in oil wells having a depth on the order of about 100 to 20,000 feet or more, are desired which maintain adequate compressive strength and density, but low permeability even at high temperature and pressure and despite contact with steam, strong brines or other corrosive substances. To meet these requirements, U.S. Pat. No. 4,069,870 to Gallus (herein incorporated by reference in its entirety) discloses addition of not more than 15 weight percent of a carbon-containing substance having a low content of volatile materials, such as lower boiling gases and liquids given off by the carbon-containing additive upon heating. Materials vaporized by the high temperatures encountered in the wellbore can disrupt the structure of the cement and thereby offset any benefits provided by the carbon-containing additive. Anthracite, calcined coke, green coke and oil shale are suitable carbon-containing materials for use in this invention.
In U.S. Pat. No. 4,114,692 to Gallus (herein incorporated by reference in its entirety) a method of introducing the carbon-containing cement into a confined annular space between the casing and the walls of the bore hole is disclosed.
To control the expense in high temperature wells of cement containing carbon additives, U.S. Pat. No. 4,200,153 to Gallus (herein incorporated by reference in its entirety) provides a method for cementing a high temperature well in which a hardenable slurry formed by admixing a carbon black additive into a conventional Portland cement in an amount between 0.01 and 1.0 weight percent is introduced into a confined space in fluid communication with a well, usually the annular space between the casing and the walls of the bore hole. Addition of such small amounts of carbon black to an oil well cement consisting of API class G or J cement is effective to impart, at minimal cost, an ultimate compressive strength of at least about 1,000 p.s.i. and an ultimate permeability less than about one millidarcy.
The lithological conditions which are compatible with the subterranean high temperatures characteristic of geothermal well sites are often weak, incompetent, or permeated by extensive fractures. For this reason, during placement of cement liners lost circulation of cement is common. Additives which increase thermal stability and compressive strength or decrease permeability of cements used in geothermal wells, such as carbon-containing additives, also increase the density of the cement, an effect which substantially contributes to the loss of cement into cracks and highly permeable zones of the formation. To overcome this disadvantage and reduce the hydrostatic pressure in the well bore during cementing, a light weight cement of high strength and thermal resistance is needed. A decrease in density has been achieved by adding such low density materials as bentonite, diatomaceous earth, and perlite. In U.S. Pat. Nos. 3,804,058 and 3,902,911 issued to Messenger, light-weight cements are made by introducing small, sealed glass or ceramic spheres as a substantial component of the cement. Although of relatively low density, these cements have relatively high water contents and relatively low compressive strengths, which generally do not exceed about 600 p.s.i.
Tinsley, in U.S. Pat. No. 4,234,344, (herein incorporated by reference in its entirety) overcomes these disadvantages by disclosing a low density cement for use in high temperature environments comprising a fine particulate crystalline silica, such as silica flour, having a particle size of less than about 10 mesh, and a light weight inorganic material, such as dispersed, stabilized gases having relatively low solubility in water--among others, hydrogen, air, oxygen and the noble gases--or an inorganic, particulate siliceous component stable at high temperature --such as sealed hollow spheres, beads of glass, and ceramic, or beads of fly ash materials. The silica and inorganic material impart thermal stability and permanent strength to the cement and reduce ultimate density or weight without sacrifice of strength.
A related problem in the cementing of high temperature geothermal wells is the premature setting of the cement. U.S. Pat. No. 3,375,873 to Mitchell (herein incorporated by reference in its entirety) solves this problem by adding ferrochrome lignosulfate to the cement slurry in an amount sufficient to prevent loss of pumpability and premature setting, usually at least 5 percent by weight of the dry Portland cement employed.
Despite these improvements in the techniques for cementing wells in highly corrosive environments, such as geothermal wells, the metal casings customarily used with cement liners are subject to severe corrosion and erosion, even when highly corrosion-resistant alloys are used. What is particularly needed is a method of cementing high temperature wells, particularly geothermal wells and wells containing high levels of hydrogen sulfide or other sulfurous compounds, so that the metal casing, which customarily serves as a conduit through which fluids are removed from the well, is protected from the corrosive and erosive action of the fluids removed from or added to the well.